1. Field of the Invention
This invention relates to a method for extrusion coating a coating composition to a length of splice jointed webs through a coating head.
2. Prior Art
A variety of systems have been employed for coating magnetic layers on supports to form magnetic recording tape. Roll coating techniques using reverse and gravure rolls are traditional. There is a transition from the roll coating techniques to coating techniques using extrusion coating heads for the reasons of higher productivity and ease of operation.
The technique of coating magnetic paint using an extrusion coating head includes forming a uniform wet film through a slit in the head, continuously transferring the wet film to a length of webs and drying the film to form magnetic recording tape.
One of problems associated with this technique is how to pass splice joints across the coating head. More particularly, where it is desired to continuously coat webs with magnetic paint, it is preferred to splice joint a series of webs, that is, connect the trailing end of one web to the leading end of a subsequent web such that the length of spliced jointed webs can be continuously fed across the head. Then the coating operation can be continued without interruption. The webs are generally connected to each other by joining the trailing end of an upstream or old web to the leading end of a downstream or new web with double adhesive tape. Although a remaining trailing portion of the upstream web is generally cut off after adhesive joining, a trailing edge portion of the upstream web which extends from the joint over a length of about 20 to 30 cm is left unadhered, just like a xe2x80x9ctailxe2x80x9d. Then as the length of webs is fed along the coating and drying sections, the free end or tail portion makes free motion relative to the adjacent web without being fixedly attached to the moving web.
In one case, the xe2x80x9ctailxe2x80x9d is present on the extrusion coating head side. If extrusion of magnetic paint is continued even during passage of the splice joint across the head, the paint is coated to the xe2x80x9ctailxe2x80x9d and then released and scattered therefrom due to the free motion of the xe2x80x9ctailxe2x80x9d induced by the running of the web, depositing on and contaminating the coating along the line or system components.
In another case, the xe2x80x9ctailxe2x80x9d is present on an opposite side to the extrusion coating head. At the splice joint that is the web overlapped portion, there is a step corresponding to the total thickness of adhesive tape and downstream web. If extrusion of magnetic paint is continued even during passage of the splice joint across the head, the paint is accumulated at the step. Since this accumulated paint cannot be fully dried in the subsequent drying step, there is left an under-dried portion leading to a coating defect. In addition, the paint can penetrate into the overlapped splice joint to cause the joint to be detached, resulting in disconnection of webs.
Therefore, where a length of splice jointed webs is continuously coated with a coating composition, it is preferred to interrupt extrusion of the composition from the head or to move the head apart from the webs during passage of the splice joint across the head so that the webs are not coated in the vicinity of the splice joint.
Several problems arise in the extrusion system designed such that extrusion of the composition from the head is interrupted during passage of the splice joint across the head. As described in Japanese Patent Application Kokai (JP-A) No. 95169/1987, since the coating head is kept in direct contact with the webs during passage of the splice joint across the head, chips of the web can deposit on the coating head, the webs can be scratched, or the webs can be electrically charged to bear electrostatic charges which can invite flash explosion in an organic solvent vapor atmosphere. Therefore, it is recommended to immediately move apart the web and the coating head when extrusion of the coating composition is interrupted.
When the coating head and the web are spaced apart during passage of the splice joint across the head, JP-A 95169/1987 employs the successive steps of interrupting extrusion of the coating composition through a slit of the head before passage of the splice joint across the head, moving the coating head and the web apart from each other, bringing the coating head and the web into mutual contact after passage of the splice joint across the head, and thereafter restarting extrusion of the coating composition from the head. This process still has some problems. Even when the discharge force for extruding the coating composition is shut off before passage of the splice joint across the head, some time will pass until extrusion of the composition is substantially interrupted. A portion of the web which is fed past the coating head during this time lag is coated to deficient thickness and must be discarded, resulting in a lowering of production yield. In addition, when extrusion of the coating composition from the head is restarted after re-contact between the web and the head, the discharge force for extrusion instantaneously acts on the web to temporarily move the web apart from the head beyond an optimum spacing, resulting in a thicker coating.
Moreover, solid dispersion type coating compositions such as magnetic paint tend to agglomerate and precipitate when allowed to stand. If such a partially agglomerated and precipitated coating composition is extruded at the restart of coating, the agglomerate or precipitate can clog the slit or the head-web spacing, resulting in coating defects including non-uniform coating thickness and longitudinal streaks on the coating surface.
Then in the method of JP-A 229572/1990, extrusion of a coating composition is not interrupted even when the web and the coating head are moved apart during passage of the splice joint across the head. The resulting thickly built-up coating is controlled at a downstream position. This method in which extrusion of the composition is not interrupted tends to form a coating of varying thickness both when the coating head and the web are moved apart and when the coating head and the web are contacted again since the head-web spacing varies at these instants. For example, when the coating head and the web are moved apart, the head-web spacing is increased to allow for formation of a locally thicker coating. The application of the composition to the web is continued until the coating head and the web are fully moved apart. When the coating head and the web are contacted again, there is some accumulation of the composition about the slit outlet of the head due to continued extrusion of the composition from the head during spacing apart of the head from the web. If the coating head and the web are contacted in this state, the composition newly extruded from the head is coated to the web together with the composition accumulated about the slit outlet, resulting in a thicker coating. Such a thicker coating is formed until the head-web spacing is stabilized.
Such unacceptable coating thickness variations lead to various product defects. Coatings thicker than the predetermined value not only lead to quality defects, but also leave wet portions due to under-drying. Under-dried paint will adhere to support rolls, guide rolls and other components along a line downstream of the drying section and transfer to the normal paint coating, leading to serious coating defects including dropouts and stains.
In this regard, JP-A 229572/1990 proposes to level such thickly coated portions by means of a smoother disposed downstream of the coating head so as to restrict the coating to the predetermined thickness. For a duration from the initial contact of the coating head with the web until the position of the coating corresponding to the point of time when the head-web spacing is stabilized goes past the smoother location, the smoother is at a close spacing to the web for restricting the thick paint coating to a predetermined uniform thickness. Thereafter, the smoother is fully spaced apart from the web, and normal coating operation is resumed.
This approach, however, often fails to achieve the predetermined thickness. The smoother is effective for reducing the thick paint coating to a thickness thinner than the coated thickness, but difficult to precisely control the coating thickness to the predetermined thickness, leaving a possibility of under-drying. Since the excessive amount of the coating composition causing a thicker coating varies every time, some extra coating composition can be left ahead of the smoother and on the web without being uniformed, particularly when a larger excess is applied. When the smoother is fully spaced apart from the web after the head-web spacing resumes a normal state, this extra mass can be transferred to the web to form a thicker coating which can be left wet after drying. These problems become serious at higher coating speeds. There is a desire to overcome these problems to provide a more efficient manufacture process.
An object of the present invention is to provide a method and apparatus for extrusion coating a magnetic paint to a length of splice jointed webs through a coating head which is designed so as to inhibit any failure resulting from an objectionable coating which can be formed upon passage of the splice joint across the head.
The present invention provides a method for extrusion coating a coating composition to a length of splice jointed webs through a coating head by feeding the length of webs across the coating head to form a coating on the webs. Each web has a splice joint at the trailing edge thereof. The coating head is moved apart from a web before the splice joint passes across the head, and brought in contact with a subsequent web after the splice joint has passed past the head. Objectionable coating portions are formed as a result of moving the coating head out of and into contact with the webs. The improvement is achieved by stripping off the objectionable coating portions from the webs at a position downstream of the head.
The composition is most often a magnetic paint.
Preferably, the stripping step is interrupted when the thickness of the coating reaches a predetermined value. The stripping step is by scraping off the objectionable coating portions from the webs. Extrusion of the composition through the coating head is continued even when the splice joint passes across the head.
In another aspect, the present invention provides an apparatus for extrusion coating a coating composition to a length of splice jointed webs, each web having a splice joint at the trailing edge thereof. The apparatus includes a head for extrusion coating the coating composition to the length of webs, a means for feeding the length of webs across the head, and a means for moving the head apart from a web before the splice joint passes across the head and bringing the head in contact with a subsequent web after the splice joint has passed past the head. Objectionable coating portions are formed as a result of moving the head out of and into contact with the webs. A means is disposed downstream of the head for stripping off the objectionable coating portions from the webs.
Preferably, the apparatus further includes a control for moving the stripping means into contact with the web for stripping when the objectionable coating portions pass across the stripping means and thereafter moving the stripping means apart from the web for interrupting the stripping step. The stripping means includes a scraping roll for scraping off the objectionable coating portions from the webs and means associated with the scraping roll for removing the scraped composition from the roll. The scraping roll is reversely rotated with respect to the feed direction of the webs. Extrusion of the composition through the head is continued even when the head is moved apart from the webs.